Production of isobutylene

ABSTRACT

Preparation of isobutylene from isobutane. Integrated process of feed preparation, dehydrogenation and purification.

[11] 3,711,569 [451 Jan. 16, 1973 1 PRODUCTION OF ISOBUTYLENE [75] -lnventors: Lloyd D. Tschopp, Humble; Terry D. Funkhouser, La Porte, both of Tex.

[73] Assignee: Petro-Tex Chemical Corporation,

Houston, Tex.

[22] Filed: Dec. 10, 1970 [21] App1.No.: 96,895

52 us. Cl. ..260/683.3 [51] Int. Cl ..C07c 5/18, C070 11/08 [58] Field of Search ..260/683.3

LIGHT GAS) LIGHT GAS 5 REMOVAL NORMAL c ISOBUTANE FEED:

DEHYDROGENATION ZONE [56] References Cited UNITED STATES PATENTS 3.4795116 11/1969 Tschopp et a1. ..260/683.3

Primary Examiner-Delbert E. Gantz Assistant ExaminerVeronica OKeefe Attorney-Baxter G. Dunaway ABSTRACT Preparation of isobutylene from isobutane. Integrated process of feed preparation, dehydrogenation and purification.

9 Claims, 1 Drawing Figure DEPROPANIZER FRACTIONATION ZONE- SOLVENT CONTACTING ZONE 20; ISOBUTYI FNl' n BUTANE n BUT ENES PRODUCTION OF ISOBUTYL'ENE BACKGROUND OF THE INVENTION This application relates to the preparation of isobutylene, preferably of a high purity, from isobutane by an integrated process which includes feed preparation, dehydrogenation and purification steps.

lsobutylene of a high purity is used for diverse applications such as one of the comonomers for butyl rubber. lsobutylene is normally separated and segregated from C, hydrocarbon fractions, obtained as petroleum process by-products and the like, by treating a C hydrocarbon stream containing isobutylene with polybasic mineral acids, particularly sulfuric acid in the range of about 55 to 70 weight percent. When the C hydrocarbon stream containing isobutylene is passed into the concentrated sulfuric acid, the isobutylene is selectively absorbed by the sulfuric acid. At the same time small amounts of other C, hydrocarbons, such as isobutane, butenes, nbutane and the like, are also absorbed in the acid. Normally, the sulfuric acid containing dissolved or entrained therein a portion of the C hydrocarbon stream as described, is first weathered, diluted and then heated to release a substantial proportion of the absorbed isobutylene. Under the process conditions as described, the purity of the isobutylene so produced may contain isobutane and butene-l as the major impurities.

Recently an improved and simplified integrated process has been'disclosed in US. Pat. No. 3,479,416. In that process a predominately isobutane feed is dehydrogenated and stripped of C and lighter gases. The remaining C stream is then solvent extracted and isobutylene is recovered from the solvent. Although this recent improvement in isobutylene preparation works to give excellent results, it does require large volumes of solvent since the entire volume of depropanized gas from the dehydrogenation is solvent extracted. An alternative integrated process is desired which will require the handling and treating smaller quantities of solvent, thus a lower capital investment and more economical operation.

It is therefore an object of this invention to provide a process that is capable of producing high purity isobutylene from a hydrocarbon stream comprising predominately isobutane. It is another object to produce isobutylene by a process which requires relatively low capital investment and which does not suffer from the disadvantages associated with processes for the sulfuric acid extraction of isobutylene from hydrocarbons, or those employing large volumes of solvent for hydrocarbon separations. Still another object of this invention is to provide a process for the simultaneously production of isobutylene and butadiene-l ,3 from a predominately isobutane hydrocarbon feed stream. These and other objects are accomplished by the invention described herein.

SUMMARY OF THE INVENTION Briefly stated, this invention is a process for the production of isobutylene which comprises feeding a hydrocarbon mixture comprising predominately isobutane and normal or straight chainfour carbon hydrocarbons to a separation zone wherein straight chain four carbon hydrocarbons are separated from isobutane to provide a hydrocarbon stream from the first separation zone having at least 50 weight percent isobutane, reacting said hydrocarbon stream from the first separation zone in a dehydrogenation reactor to form a reactor effluent comprising isobutane, isobutylene and straight chain four carbon hydrocarbons with from 0.0001 to 0.025 mol of n-butene-l per mol of isobutylene in the reactor effluent, the straight chain four carbon hydrocarbons in the reactor effluent being present in an amount greater than the weight percent of straight chain four carbon hydrocarbons fed to said reactor, fractionating the reactor effluent in a fractionating zone, removing isobutene from said fractionating zone as bottoms, contacting the various overhead from said fractionating zone in a solvent contacting zone with a solvent which selectively dissolves unsaturated hydrocarbons in preference to normal butane and isobutane, taking from said solvent contacting zone a hydrocarbon stream comprising straight chain four carbon hydrocarbons and isobutane and feeding said hydrocarbon stream to said separation zone. A preferred aspect of this process is the removal from said solvent contacting zone of a solution comprising isobutylene and solvent and separating isobutylene from said solution. Another preferred feature of the invention is the process wherein butadiene-l,3 is taken off as a side stream when the isobutylene is separated from the said solution.

The solvent extraction is considered important for the removal of olefins particularly isobutylene and butadiene from the isobutane vapors since the isobutene is to be recycled. Another aspect of the extractive distillation is the removal of butadiene which would have come off as overhead with the isobutane in the fractionating zone. Olefins frequently undergo thermal polymerization. in the dehydrogenation zone to produce higher molecular weight compounds which degrade to form tars and coke and are also more likely to undergo thermal cracking to yield lower molecular weight olefins thus are not desirable in the recycle feed. Since only the isobutane overhead is being solvent extracted in the present invention, a much smaller amount of solvent is necessary than if the total combined overhead and bottom of the fraetionating zone were solvent extracted as shown in the prior art, in order to obtain a suitably pure isobutane recycle feed.

BRIEF DESCRIPTION OF THE DRAWING One preferred method of conducting the precess of this inventionvis illustrated in the drawing. An organic stream comprising predominantly isobutane and straight chain four-carbon hydrocarbons is fed to the separation zone A. From this separation zone is taken a stream 2 having at least 50 weight percent isobutane and a minor weight percent of straight chain four carbon hydrocarbons. In dehydrogenation zone B this stream is dehydrogenated to form a reactor effluent comprising isobutane, isobutylene and straight chain four carbon hydrocarbons including n-butene-l in a particular composition. In the dehydrogenation zone B some isobutane is converted to straight chain four carbon hydrocarbons. Zones C, D and E are utilized to separate gases lighter than C hydrocarbons. Zone F is a fractionating zone with the overhead passing to a solvent contacting zone G, where the undissolved gases are returned as a liquid or gas to zone A, and a bottoms isobutylene containing stream 14 is taken off. The dissolved gases and solvent 15 from zone G are taken to a solvent separation zone H, the solvent is returned to zone G and isobutylene containing stream 18 is separated. Stream 14 or combined streams l4 and 18 may be used as such or further purified such as in zone I to separate n-butane and butene-2. A preferred embodiment of this invention is illustrated by taking a butadiene-containing stream 17 as a side stream from the solvent separation zone 1-1. This butadiene containing stream may be utilized as such or further separated by means not shown.

DESCRIPTION OF THE PREFERRED EMBODIMENT In order that those skilled in the art may more fully appreciate the nature of the invention and a method for carrying it out, it will be more specifically described in connection with the accompanying drawing which is a flow sheet of one form of the invention. The process will be illustrated by the use of particular pieces of equipment, but it is understood that a single piece of equipment may be separated into several pieces of equipment so long as the same result is achieved or conversely several pieces of equipment may be combined. Conventional auxiliary equipment such as pumps, heating and cooling means, compressors, etc., have not been shown as this type of equipment is well known to those skilled in the art.

The feed 1 to the separation zone A will comprise predominantly isobutane and straight chain C hydrocarbons. This feed stream will normally be a hydrocarbon stream and may contain other components such as hydrocarbons of two, three, five, six, etc., carbon atoms. However, the stream should still constitute predominantly four carbon hydrocarbons, and will preferably contain no greater than 40 weight percent straight chain four carbon hydrocarbons. The separation zone A may be a fractional distillation tower and may contain plates or packing. In the separation zone A straight chain four carbon hydrocarbons are separated from isobutane and are removed as a bottoms 3. For example, n-butane is separated from the isobutane. The overhead stream 2 will contain at least 50 weight percent isobutane (and preferably at least 67 weight percent isobutane based on hydrocarbons heavier than propane) and may also contain aminor weight percent of straight chain four carbon hydrocarbons, preferably no greater than 1.0 weight percent.

In the dehydrogenation zone B isobutane is dehydrogenated to isobutylene. In the dehydrogenation zone also at least a portion of the isobutane is converted to straight chain C hydrocarbons. The mechanism for this conversion is difficult to determine. For instance, isobutane may first be isomerized to n-butane which in turn may be partially dehydrogenated to normal butene, or isobutane may be first dehydrogenated to isobutylene which is then isomerized to n-butene. At any rate, the net effect is that some of the isobutane is converted to n-butene and nbutane. As a preferred embodiment from 0.1 to 1.5 or 3 mols of straight chain four carbon hydrocarbons are produced per 100 mols of isobutylene produced in the dehydrogenation zone. Also, in the dehydrogenation zone a portion of butadiene-l,3 is produced. This may be produced either from n-butane or n-butene which is fed to the dehydrogenation zone or formed in the dehydrogenation zone from other components. The dehydrogenation zone effluent 4 contains a variety of products with isobutylene being a relatively major component. The dehydrogenation zone feed, reaction conditions, catalyst, etc. are regulation such that the n-butene-l is maintained within the range of from 0.0001 to 0.025 mols of n-butene-l per mol of isobutylene in the reactor effluent and preferably no greater than 0.010 mols of n-butene-l per mol of isobutylene. Any catalyst and reaction conditions may be employed so long as the described conditions are met. The catalyst in the dehydrogenation zone may be, e.g., alumina or an oxide derived from metals of Groups IVB, VB or VIB of the Periodic Chart of the elements (e.g. Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W or mixtures thereof. The mentioned metal oxides are preferably deposited on a suitable support such as, for example, silica, silica-alumina, alumina, etc. Usually the oxides of chromium, molybdenum and vanadium deposited on alumina or magnesia are preferred as the dehydrogenation catalysts. Specific examples of such catalysts are alumina promoted with 40 percent chromium oxide, alumina promoted with 40 percent zirconium oxide, alumina promoted with 40 percent titanium dioxide, alumina promoted with 40 percent tin oxide, magnesia promoted with 20 percent molybdenum oxide, magnesia promoted with 40 percent zirconium oxide, magnesia-alumina promoted with 20 percent vanadium oxide, unsupported active chromium oxide. The preferred catalysts will contain from about 20 percent to about 30 percent by weight of the catalytic oxide of Groups IVB, VB, or VIB of the Periodic Table (Periodic Table as found on page 881 of Van Nostrand Encyclopedia of Chemical Science, 1964) supported on alumina. Although a fluid bed may be employed, the catalyst will suitably be in the form of a fixed bed.

The temperature in the dehydrogenation zone will generally be within the range of about 900 to about 1200F. The contact time and space velocity may be varied depending on other conditions with ranges such as from 0.01 minutes to about 15 minutes depending upon the type of reactor utilized. Suitable space velocities are such as from about 1 to 10 liquid hourly space velocity. Pressure is not a critical variable and the process can be operated at atmosphere, subatmosphere or super atmosphere pressure. However, the use of subatmospheric pressure results in increased yields and pressure such as from about 2 to 8 p.s.i.a. may be employed.

The effluent 4 from the dehydrogenation zone is cooled by means known to those skilled in the art such as by the use of quench, waste heat, boilers, condensers and the like. These gases are then generally compressed prior to further treatment. Zones C, D and E represent means for separating gases lighter than four carbon hydrocarbons. In these zones light gases such as CO, CO, hydrogen, methane, ethane, propane, propylene and the like are separated. A preferred method for separating these gases is set forth in the drawing. Zone C is illustrated by an absorber tower wherein a split is made between C hydrocarbons and C and-lighter gases. A reboiler may suitably be employed in the absorber to achieve the desired split between C and the C and lighter gases. The light gases 5 constitute primarily hydrogen, C0, C0 methane, ethane and ethylene. These gases may suitably be disposed of such as by burning in a boiler. However, it is sometimes advantageous to use a portion of these gases as purge gases. This is true if the dehydrogenation zone B is a cyclic type of operation wherein purge gas is employed. This purge gas may be used to purge the gas after the cycle wherein coke is burned from the catalyst. Zone C may be a conventional oil absorber and may be operated in a normal manner to achieve the desired separation. Suitable absorber oils are such as paraffinic oils. The C. hydrocarbons are absorbed in the absorber oil and taken from the absorber as stream 7 and fed to the stripper D. Absorber oil is returned to the absorber as stream 6. The stripper D is also a conventional piece of equipment wherein the absorber C hydrocarbons are separated from the absorber oil. The C and C hydrocarbons leave the stream 8 and are fed to a depropanizer E. In the depropanizer C hydrocarbons are taken off as an overhead 9 and recovered. This overhead will constitute primarily propylene and propane. The depropanizer may be a packed or plate type of fractional distillation tower. The described combination of absorber, stripper and depropanizer is the preferred embodiment according to this invention. However, other schemes may be employed such as elimination of the depropanizer or the operation of the absorber such that the split is primarily made between C and C 4 hydrocarbons. Even if a C C split is made in the absorber, a depropanizer may still be useful to separate residual C s from the C stream 9.

The stream 10 will comprise mainly C and heavier hydrocarbons and isobutylene will be a major component. According to this invention, the quantity of nbutene-l will still be within the range of from 0.0001 to 0.025 mol of n-butene per mol of isobutylene in stream 10. Stream 10 is fed to a fractionating zone F. Zone F will preferably be a distillation column. The drawing illustrates the use of a distillation column as the fractionating zone F. In zone F the stream 10 is subjected to straight distillation. The bottoms fraction 14 is a high purity isobutylene. The overhead hydrocarbon fraction is taken off as stream 13 and sent to the solvent contacting zone G. This stream 13 may suitably comprise principally isobutane, however, there is a large isobutylene concentration. In addition stream 13 will contain substantially all of the butadiene that was present in feed 10.

Zone G will preferably be an extractive distillation column but liquid-liquid extraction can satisfactorily be used under certain conditions. In Zone G the stream 13 is contacted with a solvent which selectively dissolves unsaturated hydrocarbons, particularly isobutylene and butadiene in favor of saturated hydrocarbons such as isobutane and n-butane. The undissolved hydrocarbon fraction is taken off as stream 11 and recycled to the separation zone A. This stream 11 may suitably comprise from 0.0025 to 6, preferably from 0.005 to 1 weight percent of straight chain four carbon hydrocarbons and at least 70 weight percent isobutane. Additional feed 12 from any source is also fed to the separation zone A such that the feed 1 to the separation zone is as described above.

The solvent employed in zone G can be any solvent known to those skilled in the art to make the described separation between isobutylene and saturated C hydrocarbons. A particularly preferred solvent is furfural or a furfural mixture containing water in an amount of up to 25 percent by weight of the total furfural-water mixture. Other satisfactory solvents are such as acetone, acetonitrile, dimethyl formamide, dimethyl sulphoxide, n-methyl pyrrolidone, methyl ethyl ketone and dimethyl acetamide.

The solvent containing isobutylene dissolved therein, stream 15, is fed to a solvent separating zone H where the solvent is separated as stream 16 and returned to the solvent contacting zone G.

A preferred embodiment of this invention resides in the taking of a sidestream 17 from the solvent separation column H. This sidestream is most satisfactorily taken at the point where there is a maximum concentration or bulge in the concentration of butadiene-l ,3. Stream 18 is an isobutylene stream and may be of sufficient high purity that additional purification is not required. Nevertheless, it is another feature of this invention that the isobutylene of stream 14 and l8 may be further purified by separating n-butane and the various butene-2 isomers in a factional distillation column I. High purity isobutylene 20 is taken as a product.

Another preferred embodiment of this invention is the feeding of the stream 11 as a separate stream to a fractional distillation tower such as the separation zone A. That is, the stream 11 is not mixed with feed 12. A preferred location of this feed is at a point higher in the tower than the stream 12.

The invention claimed is:

1. A process for the production of isobutylene which comprises (1) feeding a hydrocarbon mixture comprising isobutane and straight chain four carbon hydrocarbons to a first separation zone wherein straight chain four carbon hydrocarbons are separated from said mixture to provide a hydrocarbon stream from the first separating zone having at least 50 weight percent isobutane, (2) reacting said hydrocarbon stream from the first separation zone in a dehydrogenation reactor to form a reactor effluent comprising isobutane, isobutylene and straight chain four carbon hydrocarbons wherein the n-butene-l content of said straight chain four carbon hydrocarbons is such that said reactor effluent contains from 0.0001 to 0.025 moles of n-butene-l per mole of isobutylene, and the straight chain four carbon hydrocarbons in the reactor effluent are present in an amount greater than the weight percent of straight chain four carbon hydrocarbon content in said hydrocarbon stream fed to the reactor, (3) feeding the reactor effluent to a fractionating zone wherein said reactor efiluent is separated into a bottoms fraction of high purity isobutylene and an overhead fraction comprising isobutane, isobutylene and straight chain four carbon hydrocarbons, (4) withdrawing high purity isobutylene product from the fractionating zone as bottoms, (5) withdrawing said overhead fraction from said fractionating zone, (6) contacting said overhead fraction in a solvent contacting zone with a solvent which selectively dissolves unsaturated hydrocarbons in preference to normal butane and isobutane, (7) withdrawing a hydrocarbon stream comprising n-butane and isobutane from said solvent contacting zone and feeding said hydrocarbon stream to said first separating zone, (8) withdrawing a solvent mixture comprising isobutylene and solvent from said solvent contacting zone and separating isobutylene product from said solvent mixture.

2. The process according to claim 1 wherein a major portion of hydrocarbons having a boiling point lower than that of four carbon hydrocarbons are separated from the said reactor effluent prior to fractionating the reactor effluent in the fractionating zone.

3. The process according to claim 2 wherein said solvent mixture comprising isobutylene and solvent is forwarded from said solvent contacting zone to a stripping tower wherein isobutylene is stripped from said solvent mixture and a butadiene-l ,3 containing stream is taken off as a side stream from said stripping tower.

4. The process according to claim 1 wherein the said solvent comprises a member selected from the group consisting of acetone, acetonitrile, dimethyl formamide, dimethyl sulfoxide, furfural, n-methyl pyrrolidone, methyl ethyl ketone, dimethyl acetamide, and mixtures thereof with water.

5. The process of claim 1 wherein the isobutylene is fractionated to separate n-butane and butene-Z from the isobutylene.

6. A process for the production of high purity isobutylene from a hydrocarbon mixture comprising isobutane and straight chain four carbon hydrocarbons, said hydrocarbon mixture containing up to forty weight percent straight chain four carbon hydrocarbons, said process comprising (1) feeding said hydrocarbon mixture to a first separation zone wherein straight chain four carbon hydrocarbons are separated from said mixture to provide a hydrocarbon stream from the first separation zone having at least 67 weight percent isobutane based on the hydrocarbons heavier than propane and no greater than 1.0 weight percent straight chain four carbon hydrocarbons, (2) feeding said hydrocarbon stream from the first separator to a dehydrogenation reactor wherein isobutane is converted to isobutylene and straight chain four carbon hydrocarbons to form a reactor effluent comprising isobutane, isobutylene and straight chain four carbon hydrocarbons, said reactor effluent containing from 0.0001 to 0.010 moles of n-butene-l per mole of isobutylene in the reactor effluent, said straight chain four carbon hydrocarbons being produced in said reactor at a rate of 0.1 to 3 moles per 100 moles of isobutylene produced in said reactor, (3) feeding the reactor effluent to a fractionating zone wherein said reactor effluent is separated into a bottoms fraction of high purity isobutylene and an'overhead fraction comprising isobutane, isobutylene and straight chain four carbon hydrocarbons, (4) withdrawing high purity isobutylene product from said fractionating zone as bottoms, (5) contacting said overhead fraction from said fractionating zone in a solvent which selectively dissolves isobutylene in preference to n-butane and isobutane, (6) withdrawing an undissolved hydrocarbon stream comprising from 0.0025 to 6 weight percent straight chain four carbon hydrocarbons and at least 70 weight percent isobutane from said solvent contacting zone and feeding said hydrocarbon stream to said first separating isobutylene and solvent from said solvent contacting zone and feeding said solvent mixture to a second separation zone wherein isobutylene is separated from said solvent mixture.

7. A process for-the production of high purity isobutylene from a hydrocarbon mixture comprising isobutane and up to 40 percent by weight straight chain four carbon hydrocarbons, said process comprising:

1. feeding said hydrocarbon mixture to a firstvfractional distillation tower,

withdrawing an overhead hydrocarbon stream from said first fractional distillation tower, said overhead stream comprising at least 50 percent by weight isobutane, and no more than 1 percent by weight of straight chain four carbon hydrocarbons,

3. feeding said overhead hydrocarbons stream from said first fractional distillation tower to a catalytic dehydrogenation zone wherein isobutane in said overhead hydrocarbon stream is converted to isobutylene and a minor amount of straight chain hydrocarbons of four carbon atoms, said straight chain hydrocarbons being produced in an amount from 0.1 to 3 moles per lOO moles of isobutylene produced,

4. withdrawing an effluent comprising isobutane, isobutylene and straight chain hydrocarbons of four carbon atoms from said dehydrogenation zone, said effluent containing from 0.0001 to 0.025 moles of n-butene-l per mole of isobutylene,

5. feeding said efiluent from the dehydrogenation zone to an absorbing zone wherein said effluent is contacted with an absorber oil which absorbs hydrocarbons of four carbon atoms in preference to hydrocarbons of less than four carbon atoms,

6. withdrawing absorber oil rich in four carbon hydrocarbons from said absorbing zone and stripping said four carbon hydrocarbons from said absorber oil in a first stripping zone wherein said four carbon hydrocarbons are stripped from said absorber oil as stripper overhead and the lean absorber oil obtained is recycled to the absorbing zone,

7. feeding said stripper overhead to a second frac-- tional distillation tower wherein said stripper overhead is separated into a bottoms fraction essentially of isobutylene and a vaporous overhead fraction comprising isobutylene, isobutane and straight chain four carbon hydrocarbons,

8. feeding said vaporous overhead fraction from said second fractional distillation tower to a solvent contacting zone wherein said vaporous overhead fraction is contacted with a solvent which selectively dissolves isobutylene in preference to n-butane and isobutane,

. withdrawing an undissolved vaporous hydrocarbon stream comprising from 0.025 to 6 weight percent straight chain four carbon hydrocarbons and at least 78 percent isobutane from said solvent contacting zone and recycling said vaporous hydrocarbon stream to said first fractional distillation tower,

l0. withdrawing a solvent mixture from said solvent contacting zone said solvent mixture comprising isobutylene and solvent,

l l. feeding said solvent mixture to a second stripping zone wherein isobutylene is stripped as overheads from said solvent mixture and the lean solvent obtained is recycled to said solvent contacting zone,

12. combining said isobutylene from the bottoms fraction of said second fractional distillation column with said isobutylene obtained as overheads from said second stripping zone and fractionally distilling the combined isobutylene to recover a product of at least 99 weight percent 10 isobutylene.

8. A process as claimed in claim 7 wherein the catalyst employed in said catalystic dehydrogenation 

2. The process according to claim 1 wherein a major portion of hydrocarbons having a boiling point lower than that of four carbon hydrocarbons are separated from the said reactor effluent prior to fractionating the reactor effluent in the fractionating zone.
 2. withdrawing an overhead hydrocarbon stream from said first fractional distillation tower, said overhead stream comprising at least 50 percent by weight isobutane, and no more than 1 percent by weight of straight chain four carbon hydrocarbons,
 3. feeding said overhead hydrocarbons stream from said first fractional distillation tower to a catalytic dehydrogenation zone wherein isobutane in said overhead hydrocarbon stream is converted to isobutylene and a minor amount of straight chain hydrocarbons of four carbon atoms, said straight chain hydrocarbons being produced in an amount from 0.1 to 3 moles per 100 moles of isobutylene produced,
 3. The process according to claim 2 wherein said solvent mixture comprising isobutylene and solvent is forwarded from said solvent contacting zone to a stripping tower wherein isobutylene is stripped from said solvent mixture and a butadiene-1,3 containing stream is taken off as a side stream from said stripping tower.
 4. The process according to claim 1 wherein the said solvent comprises a member selected from the group consisting of acetone, acetonitrile, dimethyl formamide, dimethyl sulfoxide, furfural, n-methyl pyrrolidone, methyl ethyl ketone, dimethyl acetamide, and mixtures thereof with water.
 4. withdrawing an effluent comprising isobutane, isobutylene and straight chain hydrocarbons of four carbon atoms from said dehydrogenation zone, said effluent containing from 0.0001 to 0.025 moles of n-butene-1 per mole of isobutylene,
 5. feeding said effluent from the dehydrogenation zone to an absorbing zone wherein said effluent is contacted with an absorber oil which absorbs hydrocarbons of four carbon atoms in preference to hydrocarbons of less than four carbon atoms,
 5. The process of claim 1 wherein the isobutylene is fractionated to separate n-butane and butene-2 from the isobutylene.
 6. withdrawing absorber oil rich in four carbon hydrocarbons from said absorbing zone and stripping said four carbon hydrocarbons from said absorber oil in a first stripping zone wherein said four carbon hydrocarbons are stripped from said absorber oil as stripper overhead and the lean absorber oil obtained is recycled to the absorbing zone,
 6. A process for the production of high purity isobutylene from a hydrocarbon mixture comprising isobutane and straight chain four carbon hydrocarbons, said hydrocarbon mixture containing up to forty weight percent straight chain four carbon hydrocarbons, said process comprising (1) feeding said hydrocarbon mixture to a first separation zone wherein straight chain four carbon hydrocarbons are separated from said mixture to provide a hydrocarbon stream from the first separation zone having at least 67 weight percent isobutane based on the hydrocarbons heavier than propane and no greater than 1.0 weight percent straight chain four carbon hydrocarbons, (2) feeding said hydrocarbon stream from the first separator to a dehydrogenation reactor wherein isobutane is converted to isobutylene and straight chain four carbon hydrocarbons to form a reactor effluent comprising isobutane, isobutylene and straight chain four carbon hydrocarbons, said reactor effluent containing from 0.0001 to 0.010 moles of n-butene-1 per mole of isobutylene in the reactor effluent, said straight chain four carbon hydrocarbons being produced in said reactor at a rate of 0.1 to 3 moles per 100 moles of isobutylene produced in said reactor, (3) feeding the reactor effluent to a fractionating zone wherein said reactor effluent is separated into a bottoms fraction of high purity isobutylene and an overhead fraction comprising isobutane, isobutylene and straight chain four carbon hydrocarbons, (4) withdrawing high purity isobutylene product from said fractionating zone as bottoms, (5) contacting said overhead fraction from said fractionating zone in a solvent which selectively dissolves isobutylene in preference to n-butane and isobutane, (6) withdrawing an undissolved hydrocarbon stream comprising from 0.0025 to 6 weight percent straight chain four carbon hydrocarbons and at least 70 weight percent isobutane from said solvent contacting zone and feeding said hydrocarbon stream to said first separating zone (7) withdrawing a solvent mixture comprising isobutylene and solvent from said solvent contacting zone and feeding said solvent mixture to a second separation zone wherein isobutylene is separated from said solvent mixture.
 7. A process for the production of high purity isobutylene from a hydrocarbon mixture comprising isobutane and up to 40 percent by weight straight chain four carbon hydrocarbons, said process comprising:
 7. feeding said stripper overhead to a second fractional distillation tower wherein said stripper overhead is separated into a bottoms fraction essentially of isobutylene and a vaporous overhead fraction comprising isobutylene, isobutane and straight chain four carbon hydrocarbons,
 8. feeding said vaporous overhead fraction from said second fractional distillation tower to a solvent contacting zone wherein said vaporous overhead fraction is contacted with a solvent which selectively dissolves isobutylene in preference to n-butane and isobutane,
 8. A process as claimed in claim 7 wherein the catalyst employed in said catalystic dehydrogenation zone comprises chromium oxide, said absorber oil is a paraffin oil and said solvent employed in the solvent contacting zone is a member selected from the group consisting of acetone, acetonitrile, diemthyl formamide, dimethyl sulfoxide, furfural, n-methyl pyrrolidone, methyl ethyl ketone, dimethyl acetamide and mixtures thereof with water.
 9. A process as claimed in claim 8 wherein the hydrocarbon mixture fed to said first fractional distillation tower contains no greater than 20 weight percent straight chain four carbon hydrocarbons and at least 70 weight percent isobutane.
 9. withdrawing an undissolved vaporous hydrocarbon stream comprising from 0.025 to 6 weight percent straight chain four carbon hydrocarbons and at least 78 percent isobutane from said solvent contacting zone and recycling said vaporous hydrocarbon stream to said first fractional distillation tower,
 10. withdrawing a solvent mixture from said solvent contacting zone said solvent mixture comprising isobutylene and solvent,
 11. feeding said solvent mixture to a second stripping zone wherein isobutylene is stripped as overheads from said solvent mixture and the lean solvent obtained is recycled to said solvent contacting zone,
 12. combining said isobutylene from the bottoms fraction of said second fractional distillation column With said isobutylene obtained as overheads from said second stripping zone and fractionally distilling the combined isobutylene to recover a product of at least 99 weight percent isobutylene. 